This document claims priority and contains subject matter related to Japanese Patent Application No. 11-147729 filed in the Japanese Patent Office on May 27, 1999, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile, a printer, or similar image forming apparatus, and more particularly relates to an image forming apparatus that can form a smooth image with inconspicuous granularity.
2. Discussion of the Background
In a known image forming apparatus, to form a toner image on a recording member such as a transfer sheet, first an electrostatic latent image is formed on a surface of a photoreceptor, serving as an image bearing member by irradiating the surface of the photoreceptor, which is uniformly charged, with a light beam emitted from an LD (Laser Diode) or an LED (Light Emitting Diode). Subsequently, the electrostatic latent image on the photoreceptor is developed with toner, and then the toner image is transferred to the recording member. The transferred toner image is fixed on the recording member by a fusing device.
In the above-described image forming apparatus, a gradation representation method is known wherein an electrostatic latent image of a high density image portion of an image is formed on the photoreceptor by emitting a light beam which is intense enough to fully attenuate the potential of the photoreceptor (hereinafter referred to as a saturation writing). Further, for a halftone (intermediate density) image portion and a highlight (low density) image portion, an electrostatic latent image having an intermediate potential is formed on the photoreceptor by changing the intensity and irradiating time of the light beam. In the image forming apparatus employing the above-described gradation representation method, in order to fully reproduce the gradation and the highlight image portion, a method may be generally employed wherein an electrostatic latent image of an intermediate potential is formed on the photoreceptor by changing the intensity and irradiating time of the light beam for each pixel, i.e., a basic unit forming an image (hereinafter referred to as a one dot multilevel process).
However, because static electricity is used to form an image in an electrophotographic recording process, an image quality, especially the gradation characteristic of a halftone intermediate image portion, may vary depending on the characteristic changes of the photoreceptor and the developer caused by environmental conditions (e.g., temperature and humidity) and a lapse of time.
Another gradation representation method is known wherein respective electrostatic latent images of high density, intermediate density, and highlight (low density) image portions are all formed by the above-described saturation writing, and the gradation is represented by changing a density of toner adhering pixels of the image forming pixels per unit area of the image (hereinafter referred to as a bilevel process). FIG. 4 is a graph illustrating a relation between the writing energy of a light beam and the surface potential of a photoreceptor. A characteristic line a1 represents a distribution of the writing energy of a light beam, and a characteristic line a2 represents a distribution of the surface potential of a dot of a pixel on the surface of the photoreceptor which is irradiated with the light beam.
Specifically, the surface of the photoreceptor is charged to a predetermined level, i.e., charged potential Vd. As illustrated in FIG. 4, when the surface of the photoreceptor is irradiated with the light beam which has maximum energy at the center of the curvature of the characteristic line al and is intense enough to attenuate the surface potential of the photoreceptor to approximately 0V, the surface potential of the center portion of the dot of the pixel on the photoreceptor is attenuated to approximately 0V. The diameter of the dot developed with toner is substantially the same as a diameter of a corresponding portion of the photoreceptor where the absolute value of the surface potential of the photoreceptor is smaller than an absolute value of a starting potential for developing.
If the above-described bilevel process is employed as the gradation representation method, an image quality can be more stable because the intermediate potential is not used for halftone and highlight image portions.
However, it is known that in the bilevel process, the toner adhering pixels are likely to be sharply outlined in a toner image formed on a transfer member, and thereby the image is likely to produce an impression that the image surface is granular (not smooth). In the electrophotographic apparatus, the image quality in the bilevel process is generally inferior to that in the one dot multilevel process in respect of smoothness of the image.
The present invention has been made in view of the above-discussed and other problems, and an object of the present invention is to address these problems.
The preferred embodiment of the present invention provides a novel image forming apparatus that can form a smooth image with inconspicuous granularity.
These objects and others are achieved according to the present invention by providing a novel image forming apparatus including an image bearing member, and an image processing device configured to process image data of an image including pixels forming the image such that a gradation of the image is represented by a density of toner adhering pixels of the image forming pixels per unit area of the image. The image forming apparatus further includes a latent image forming device configured to form an electrostatic latent image of the image including the image forming pixels on the image bearing member in accordance with the image data processed by the image processing device, a developing device configured to visualize the electrostatic latent image of the image on the image bearing member to a toner image, and a transfer device configured to transfer the toner image on the image bearing member to a transfer member.
A diameter of a pixel toner image of each toner adhering pixel of the toner image on the image bearing member is in a range of 20 xcexcm to 50 xcexcm, and the transfer device includes a primary transfer member, a primary transfer device configured to transfer the toner image on the image bearing member to the primary transfer member, and a secondary transfer device configured to transfer the toner image on the primary transfer member to the transfer member.
A toner scattering is generated around a nucleus portion of the pixel toner image of each toner adhering pixel of the toner image on the transfer member, and a diameter of the pixel toner image is 1.5 times or more a diameter of the nucleus portion of the pixel toner image on the transfer member.
According to the present invention, the image forming apparatus includes an image processing device configured to process image data of an image including pixels forming the image such that a gradation of the image is represented by a distance between toner adhering pixels of the image forming pixels.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.